The mechanism by which tumors are induced by N-nitroso-carcinogens will be studied. The interaction of labeled dimethylnitrosamine and N-methyl-N-nitrosourea and their ethyl analogs with DNA in target tissues of rodents will be measured as a function of dose of carcinogen and in a variety of physiological conditions. Certain organs are much more susceptible to the carcinogenic stimulus than others and this susceptibility varies with the physiological state of the animal and the dose of carcinogen employed. The hypothesis that the initial carcinogenic stimulus produced by these agents results from the alkylation of DNA at oxygen atoms particularly the O6-position of guanine and that the difference in susceptibility between different tissues is due to a difference in the ability to remove these alkylated products from DNA before cell division will be studied. The role of physiological factors including hormones which appear to control the rate of this removal will be investigated. The enzymatic mechanism resulting in the removal of O6-alkylguanine from DNA in various mammalian cells will be investigated using a cell-free system isolated from rat liver. The mechanism by which this activity is inhibited by high doses of N-nitroso-carcinogens will be studied and the importance of this inhibition in carcinogenesis evaluated. The further metabolism of alkylated purines and pyrimidines and their possible reincorporation into DNA will be studied. Isolated rat hepatocytes will be tested to determine whether they provide a suitable system to investigate the interaction of nitrosamines with DNA and its subsequent repair. There is widespread human exposure to low levels of N-nitroso-carcinogens. The enzymatic process capable of removing the products of interaction with DNA responsible for tumor induction may counteract the effects of this exposure. Knowledge of the physiological factors affecting this process might enable groups at high risk to be identified.